Question

首次使用Boost，并尝试了解如何使用boost::variant。请在下面找到示例程序和错误消息（GCC v5.3.0，-std = c ++ 14）

#include <vector>
#include <iostream>
#include <sstream>

#include <boost/variant.hpp>

using int_tree_t =
    boost::make_recursive_variant <
      int,
      std::vector<boost::recursive_variant_>>;

auto operator << (std::ostream& out, const int_tree_t& tree) ->
std::ostream&
{
  struct stringify :
    public boost::static_visitor <std::string>
  {
  public:
    auto operator () (int i) const ->
    std::string
    {
      return std::to_string (i);
    }

    auto operator () (const std::vector<int_tree_t>& vec) const ->
    std::string
    {
      std::stringstream ss;
      ss << "(";

      auto it = std::begin (vec);

      if (!vec.empty ()) {
        ss << *it;
      }

      for (; it != std::end (vec); ++it) {
        ss << " " << *it;
      }

      ss << ")";

      return ss.str ();
    }
  };

  std::stringstream ss;
  ss.flags (out.flags ());

  ss << boost::apply_visitor (stringify (), tree);

  return (out << ss.str ());
}

错误讯息：

In file included from /usr/local/include/boost/variant/apply_visitor.hpp:16:0,
                 from /usr/local/include/boost/variant/detail/hash_variant.hpp:23,
                 from /usr/local/include/boost/variant/variant.hpp:34,
                 from /usr/local/include/boost/variant.hpp:17,
                 from test2.cpp:5:
/usr/local/include/boost/variant/detail/apply_visitor_unary.hpp: In instantiation of ‘typename Visitor::result_type boost::apply_visitor(const Visitor&, Visitable&) [with Visitor = operator<<(std::ostream&, const int_tree_t&)::stringify; Visitable = const boost::make_recursive_variant<int, std::vector<boost::recursive_variant_, std::allocator<boost::recursive_variant_> > >; typename Visitor::result_type = std::__cxx11::basic_string<char>]’:
test2.cpp:50:43:   required from here
/usr/local/include/boost/variant/detail/apply_visitor_unary.hpp:84:43: error: ‘const struct boost::make_recursive_variant<int, std::vector<boost::recursive_variant_, std::allocator<boost::recursive_variant_> > >’ has no member named ‘apply_visitor’
     return visitable.apply_visitor(visitor);
                                           ^

如果我理解正确，boost::make_recursive_variant应该创建一个与boost::variant相同的类型...但是错误消息似乎表明它没有apply_visitor功能，这是访问所必需的。我已经阅读了[this（http://www.boost.org/doc/libs/1_60_0/doc/html/variant/tutorial.html）页面，但没有任何关于必须使用带有递归变体的“特殊”访问模式。

我该如何编写这个程序？

Answer 1

make_recursive_variant是一个元函数，它本身不是一个类型。所以你必须评估元函数：

否则，您使用元函数类型而不是变体类型。

boost :: make_recursive_variant没有名为'apply_visitor'的成员

1 个答案: